Large bandwidth optical parametric amplifier

ABSTRACT

It is proposed to amplify WDM optical signals using optical parametric amplification by apply an appropriate separation of those optical signals according to their carrier angular frequencies. These angular frequencies are within the two principle amplification bands defined by the used non-linear optical medium for optical parametric amplification. It is advantageously proposed to launch into the non-linear optical medium in one direction the optical signals of carrier frequencies within the one amplification band and in the opposite counter-propagating direction the optical signals of carrier angular frequencies within the other amplification band. The required pump radiation is being launched co-linearly with the respective optical signals to be amplified.

TECHNICAL FIELD

The present invention relates to a method for amplifying wavelengthsdivision multiplexed optical signals by using an optical devicecomprising a non-linear optical medium for optical parametricamplification OPA of the optical signal when transmitted co-linearlywith some pump radiation, the optical signals having carrier angularfrequencies within the two principle amplification bands defined by theused optical parametric amplification. Furthermore, the presentinvention is also related to an optical device comprising an opticalcircuit with at one arm a non-linear optical medium for generatingamplified optical signals using OPA and wavelengths specific couplerswith three ports to distribute optical signals according to theircarrier angular frequency property to different arms of the opticalcircuit. The invention is based on a priority application EP 04 292118.9 which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Optical fiber technology is currently applied in communication systemsto transfer information, e.g. voice signals and data signals, over along distance as optical signals. Over such long distance, however, thestrength and quality of a transmitted optical signal diminish.Accordingly, techniques have been developed to regenerate or amplifyoptical signals as they propagate along an optical fiber.

One well known amplifying technique called Raman amplification exploitsan effect called Raman scattering to amplify an incoming informationbearing optical signal. Raman scattering describes the interaction oflight with molecular vibration of the material for which the lightpropagates. And Raman amplification is based on a nonlinear effect ofsilica which is the main element of optical fibers. When exposed to aradiation, the material of the fiber absorbs a part of the energy (whichcorresponds to vibrational states of the molecular structure). Incidentlight scattered by molecules experience a downshift in frequency fromthe power bearing optical pump radiation. This downshift in frequency(or increase in wavelengths) from the pump radiation is referred to asthe Stokes shift and the corresponding scattered light as Stokes-line.In fact, at least a second scattered light can be measured in asymmetric way respective to the optical pump radiation (thereforecorresponding to an upshift in frequency) which is referred as theanti-Stokes-line and considered usually as negligible. The extend of theshift and the shape of the Raman gain curve is determined by themolecular vibrational frequency modes of the transmission medium. Inamorphous materials, such as silica, molecular vibrational frequenciesspread into bands which overlap and provide a broadband wide gain curve.The efficiency or the characteristics of the Raman effect can beimproved by introducing dopants in the fiber like Germanium orPhosphorus. An amplification based on the use of such amorphous materialis usually called parametric amplification and is nowadays quite popularfor amplifying optical signals partly due to its promise of noiselesssignal regeneration.

A well promising amplification is based on the third order opticalparametric effect obtained by coupling a signal wave (optical signal ofcarrier angular frequency ω_(s)) with pump radiations (of carrierangular frequencies ω_(p1), ω_(p2)) in an non-optical medium andpropagating therefrom to induce the third order optical parametriceffect. The optical signal is thus amplified and through a four-wavemixing process (shortened to FWM hereinbelow), a new radiation having acarrier angular frequency ω_(f) is generated. Here, the carrier angularfrequencies ω_(s), ω_(p1), ω_(p2), ω_(f) of the optical signal, the pumpradiation and the FWM generated radiation are governed by the law ofconservation of energy as expressed in the following equation:ω_(s)+ω_(f)=ω_(p1)+ω_(p2).

The generated FWM radiation has a mirror symmetry with the spectrum ofoptical radiation with respect to the carrier angular frequency(ω_(p1)+ω_(p2))/2, and functions also as the optical phase conjugationradiation for the optical signal. When an optical circuit comprisingsuch a non-linear optical medium is to be used as an FWM radiationgenerator, it is necessary to pack the carrier angular frequenciesω_(s), ω_(p1) and ω_(p2) to increase the conversion gain (expressed asFWM radiation intensity/optical signal intensity) of optical signal toFWM radiation, as well as increase the pump radiation intensity.Similarly, when such optical circuit is to be used as an opticalparametric amplifier (shortened to OPA hereinbelow), it is necessary topack the carrier angular frequencies ω_(s), ω_(p1), ω_(p2) and increasethe pump radiation intensity to increase the amplification gain of theoptical signal. When the requirement is to amplify optical signals thenonly latter configuration will be used. In that case, only the amplifiedoptical signals at carrier frequency ω_(s) will have to be collected atthe output of the optical circuit. All the other radiations i.e. pumpradiations ω_(p1), ω_(p2) and the generated FWM radiation ω_(f) willhave somehow to be filtered out to avoid any cross-talk.

It should be noted that, in addition to the generated FWM radiation of acarrier radiation frequency ω_(p1)+ω_(p2)−ω_(s) (above defined as ω_(f)generated in the non-linear optical medium, unwanted FWM radiations ofcarrier frequencies 2ω_(p1)−ω_(s), 2ω_(p2)−ω_(s) are produced by thedegenerated pump radiations at a spacing |ω_(p1)−ω_(p2)| per differentoptical signals ω_(s). for optical signals of different wavelengths asused in wavelength division multiplexed WDM optical signals then iftheir carrier frequencies and the pump radiation satisfy the expressionω_(p1)<ω_(sj)<ω_(p2) (j=1, 2, . . . N) the FWM radiation F_(j) ofcarrier frequency ω_(p1)+ω_(p2)−ω_(sj) excited by the non-degeneratepump radiations is generated between the frequencies ω_(p1), ω_(p2). Andthe unwanted FWM radiations called also idlers excited by thedegenerated pump radiations of carrier frequencies 2ω_(p1)−ω_(sj),2ω_(p2)−ω_(sj) are generated in a range outside the frequencies ω_(p1),ω_(p2) (within “secondary” amplification bands). For OPA, not only theFWM radiation ω_(fj) but also the idlers will have to be filtered out tocollect only the amplified optical signals at carrier frequenciesω_(sj).

It exists a limiting case of OPA which is of interest for theamplification of optical signals and corresponds to the case of the useof a single pump radiation. Such limiting case, called degenerated OPA,is simply obtained when setting the two pump radiations of carrierfrequencies ω_(p1), ω_(p2) to be equal. The previously described idlersobtained at 2ω_(p-s) ops will coincide with the main FWM radiation F ofcarrier frequency ω_(f) in that limiting case of a single used pumpradiation of carrier frequency ω_(p). In fact, the optical signal ofcarrier frequency ω_(s) and the FWM generated radiation of carrierfrequency ω_(f) correspond respectively to the Stokes line and to theanti-Stokes line symmetric between each other respective to the pumpradiation of carrier frequency ω_(p). It must be noticed that in thedegenerated OPA the generated FWM radiation corresponding to theanti-Stokes line is of an order of the amplified optical signal and istherefore no more negligible. In fact, it is very important to filterout such generated FWM radiation when collecting the amplified opticalsignals to avoid any cross-talk or mismatch. The two amplification bands(principle amplification bands) defined in the two pump OPA in theintervals [ω_(p1), (ω_(p1)+ω_(p2))/2] and [(ω_(p1)+ω_(p2))/2, ω_(p2)] ifω_(p1)<ω_(p2) will now be located on the right and the left of the usedpump radiation of carrier frequency ω_(p).

In “Interleaver-Based Method for Full Utilization of the Bandwidth ofFiber Optical Parametric Amplifiers and Wavelength Converters” from M.Marhic et al., ThK4, OFC 2003, is presented a method using two parallelfiber optical parametric amplifiers (OPAs) and two interleavers. Suchsystem is used to either amplify or spectrally invert a broad spectrum.Indeed, fiber OPAs can exhibit gain bandwidth of several hundrednanometers. However, if a densely populated wavelength divisionmultiplexed WDM spectrum is presented at the input, covering the entireOPA bandwidth, then at the output the signal and idler spectra overlapcompletely, and it is not possible to place the idlers in gaps betweenthe signals. Without modification, this fundamental problem limits theusable width of a WDM spectrum to about half the potential full OPAbandwidth. To use the full OPA bandwidth, one needs to use filters toseparate the signals into two groups, and amplify them separately.

The method presented in the above paper ThK4 at OFC 2003 is based on theuse of an interleaver, to separate even and odd channels. These are thenamplified separately, and recombined by another interleaver. With afour-port second interleaver, the signal spectrum and the idler spectrumare simultaneously available from the two output ports. The methodrequires that the carriers and the interleavers be precisely alignedwith a common ITU grid. Therefore, once the architecture is defined andthe interleaver chosen, channels spacing cannot be changed and systemupgraded. Moreover, the number of channels is fixed which is a veryserious drawback of such solution.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention topropose a method and provide an optical device for amplifying wavelengthdivision multiplexed optical signals using optical parametricamplification in an optimized way while guaranteeing a maximumflexibility particularly in case of a possible upgrade.

This object is achieved in accordance with the invention by anappropriate separation of the optical signals to be amplified accordingto their carrier angular frequencies. These angular frequencies arewithin the two principle amplification bands defined by the usednon-linear optical medium for optical parametric amplification. It isadvantageously proposed to launch into the non-linear optical medium inone direction the optical signals of carrier frequencies within the oneamplification band and in the opposite counter-propagating direction theoptical signals of carrier angular frequencies within the otheramplification band. The required pump radiation is being launchedco-linearly with the respective optical signals to be amplified.

It is also proposed to use wavelengths specific couplers with threeports for the optical device to separate optical signals into differentpaths of an optical circuit comprising the non-linear optical medium.This separation will be performed according to the carrier angularfrequency property from an optical signals if belonging within the firstor the second principle amplification bands. This will advantageouslyallow to collect at the output of the optical device the amplifiedoptical signals while the unwanted produced four wave mixing FWMradiations within the two principal amplification bands leaving theoptical device by its input side.

It is also proposed to use an optical device according to the inventioncomprising four couplers defining an eight-like optical circuit made outof two closed paths. One of that paths is connected to the input of theoptical device by one of such couplers, the other being connected to theoutput of the optical device by another coupler. Both closed paths havea common arm comprising the non-linear optical medium used for the OPAwhile being interconnected with the two closed paths at both ends byrespective coupler to collect at the output of the optical device theoptical amplified signals while the unwanted produced idlers within thetwo principal amplification bands leave to its input.

In the case of degenerated OPA i.e. use of a single pump radiation, itis of advantage that the used couplers for the optical device accordingto the invention are defined by a wavelength cut-off for the separationlimit being closed but not equal to the required pump radiation.

Advantageous developments of the invention are described in thedependent claims, the following description and the drawings.

DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention will now be explained furtherwith the reference to the attached drawings in which:

FIG. 1 is sketched a schematic view of an optical circuit according tothe invention with the propagation paths of optical signals belonging tothe two principle amplification bands;

FIG. 2 is shown the optical circuit according to FIG. 1 with thepropagation paths of the pump radiation in the degenerated OPA case;

FIG. 3 shows a schematically view of an alternative architecture for anoptical circuit in the case of the degenerated OPA according to theinvention;

FIG. 4 is described an implementation of the optical circuit into theoptical device for degenerated OPA according to the invention;

FIG. 5 is sketched the propagation paths of the used two pump radiationsalong the optical circuit according to FIG. 1;

FIG. 6 shows an alternative architecture when using two pump radiations;

FIG. 7 is shown an architecture of the optical circuit when used withtwo pump radiations according to the invention;

FIGS. 8 a and b are described the location of the two principle bonds inrelation with the carrier angular frequencies of the used pumpradiations.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

On FIG. 1 is shown an optical circuit 1 according to the invention. Itcomprises four optical couplers 5 to 8 each with three ports anddefining an eight-like optical circuit made out of two closed paths 14,15. One closed path 14 is connected to the input 2 of the optical deviceby the coupler 5. The other closed path 15 is connected to the output 3of the optical device by another coupler 6. Both closed paths 14, 15have a common arm 9 comprising the non-linear optical medium 4 in formof a fiber OPA. This path 9 is interconnected with the two closed paths14, 15 at both ends by a respective coupler 7, 8.

The four used couplers 5 to 8 are wavelength specific couplers,preferably but not exclusively chosen as multiplexer. They are definedsuch that the optical signals to be amplified are separated into twodifferent paths of the optical circuit 1. The separation is performedaccording to the carrier angular frequency property from the opticalsignals if belonging within the first or the second principleamplification bands defined by the used optical parametricamplification.

On FIG. 1 is shown also the typical propagation paths of optical signalsλ_(s1) (having carrier angular frequency ω_(s1)) and λ_(s2) (havingcarrier angular frequency ω_(s2)) belonging to respectively the twoprinciple amplification bands defined by the ranges [ω_(p1),(ω_(p1)+ω_(p2))/2] and [(ω_(p1)+ω_(p2))/2, ω_(p2)] with ω_(p1) andω_(p2) angular frequencies of the two used pump radiations andω_(p1)<ω_(p2) (see energy spectrum on FIG. 8 a in carrier angularfrequency scale). The four used wavelengths specific couplers 5 to 8have same wavelength cut-off λ_(c) (carrier angular frequency ω_(c) forthe separation limit of the two principle amplification bands. In thatcase and according to the invention, the optical signals λ_(s1), λ_(s2)are separated by the coupler 5 at the input 2 into two different paths10 and 11 of the first closed path 14. Both paths 10, 11 lead each to acoupler 7, 8 connected to the respective end of the path 9 comprisingthe non-linear optical medium 4. In such a way the optical signal λ_(s1)(ω_(s1)) within the one amplification band is launched by the coupler 8into the fiber OPA (non-linear optical medium 4) on one direction whilethe optical signals λ_(s2) (ω_(s2)) within the other amplification bandby the coupler 7 in the opposite counter-propagating direction. Theoptical signals λ_(s1), λ_(s2), after being amplified by OPA takingplace at the fiber 4 will leave the first closed path 14 to be directedat the two paths 13, 12 leading to the coupler 6 of the second closedpath 15 connected to the output 3 of the optical device. The requiredpump radiation are not shown on FIG. 1 but are launched co-linearly withthe respective optical signals for the amplification to take place atthe non-linear optical medium 4.

On FIG. 1 is shown the optical device used according to the invention inthe case of the degenerated OPA i.e. the one pump parametric amplifierwith the use of a single pump radiation of carrier angular frequencyω_(p) (limiting case corresponding to ω_(p1)=ω_(p2)=ω_(p) and the twoprincipal amplification bands defined now respectively by ω_(s)<ω_(p)and ω_(s)>ω_(p)). In that case, the cut-off wavelength of the couplers 5to 8 must be a little longer or shorter (about 2 nm) than the pumpwavelength (proportional to the inverse of ω_(p)). Then, the pumpradiation propagates in the amplifier either like optical signalsbelonging to the first band or the second band. Furthermore, theunwanted Four-Waves Mixing FWM radiations (idlers) are generated withthe carrier angular frequency corresponding to 2ω_(p)−ω_(s) for theoptical signal of carrier angular frequency ω_(s).

Such unwanted FWM waves are generated symmetrically respective to thepump radiation of carrier angular frequency (op like an anti-Stokesline. In that case, for an optical signal λ_(s1) with carrier angularfrequency within the first amplification bond defined by ω<ω_(p) anidler λ_(i1) is generated with carrier angular frequency within thesecond principle amplification band defined by ω>ω_(p) (see energyspectrum on FIG. 8 b in carrier angular frequency scale). In a same way,an optical signal λ_(s2) with carrier angular frequency within thesecond principle amplification band generates an idler λ_(i2) withcarrier angular frequency within the first principle amplification band.The use of an optical device with an optical circuit made out of thefour couplers 5 to 8 will have the advantage as shown on FIG. 1 to leadthe unwanted produced FWM radiations i.e. on FIG. 1 λ_(i1) and λ_(i2) tothe input 2 of the optical device. In such a way, it is possible tocollect at the output 3 of the optical device solely the amplifiedoptical signals λ_(s1) and λ_(s2). Therefore, there is no need of anyfilters for the idlers at the output and no cross-talks or mismatchbetween the idlers and the optical signals can take place.

The required pump radiation, in the case of a degenerated OPA only asingle radiation at carrier angular frequency ω_(p), will have to beinjected in the non-linear optical medium 4 in both directionco-linearly with the optical signals to be amplified. For that, twodifferent implementation can be applied.

On FIG. 1 is shown a first implementation by dividing the pump radiationin two and injecting it into the optical circuit 1 both by its input 2and its output 3. In the case that the cut-off wavelength of thecouplers is to be chosen that the pump radiation propagates like anoptical signals with carrier angular frequency within the first band,the pump radiation injected at the input 2 will follow in the firstclosed path 14 the arm 10 to be injected into the arm 9 comprising thefiber OPA in one direction while the pump radiation injected at theoutput 3 will follow the arm 12 of the closed path 15 to be injectedinto the arm 9 in a counter-propagating way.

On FIG. 3 is shown a similar optical circuit 1 comprising this time asupplementary mirror 25 on the arm 9 together with the non-linearoptical medium 4. Such mirror can be chosen as a fiber Bragg gratingoptimized to reflect the pump radiation of carrier angular frequencyω_(p). In that case, it is no more necessary to divide the pumpradiation in two since the pump radiation can now be injected only fromone side e.g. from the input of the optical device as shown on FIG. 3.

In the case of symmetric injection of the pump radiation as shown onFIG. 2, an architecture for the optical device as sketched on FIG. 4 canbe used. The device D couples pump radiation and channels of the WDMoptical signals. Between the two devices D is placed the optical circuit1. The full band, except for the pump radiation should be transmittedfrom port a to port c (and reciprocally). Only the pump radiation shouldbe transmitted from port b to port c (and reciprocally). An opticaldevice according to such architecture could be realized by using thinfilm technology as in conventional multiplexer but, replacing the waveband splitter function by a notch filter or Fabry-Perot type functioncentered at the wavelength corresponding to the pump radiation andreflecting only that corresponding pump radiation. Similarly, it ispossible to use a filter which reflects all channels except the pumpradiation.

For OPA with two different pump radiations of carrier angularfrequencies ω_(p1) and ω_(p2) and if ω_(p1)<ω_(p2) than the twoprinciple amplification bands are defined in the range [ω_(p1),(ω_(p1)+ω_(p2))/2] and [(ω_(p1)+ω_(p2))/2, ω_(p2)]. If the WDM channelsof the optical signals and the used pump radiations satisfy theexpression ω_(p1)<ω_(sj)<ω_(p2) (j equal 1, 2, . . . , N) for N channelsthen the optical signals will belong to the first or secondamplification band if respectively they are less or greater than(ω_(p1)+ω_(p2))/2. To each amplified optical signal ω_(sj) (λ_(sj)) willbe generated a FWM radiation at ω_(p1)+ω_(p2)−ω_(sj) i.e. a FWMradiation within the second amplification band if ω_(sj) within thefirst one and vice-versa. These correspond to the idlers in thedegenerated case. On top of that, supplementary unwanted couple ofidlers are generated at carrier angular frequencies (2ω_(p1)−ω_(sj),2ω_(p2)−ω_(sj)), i.e. in a range outside the two amplification bands (atsecondary amplification bands). On FIG. 8 a is shown the respectivelocation on the energy spectrum (carrier angular frequency scale) of thecouple of idlers (ω_(is1p1), ω_(is1p2)) and (ω_(is2p1), ω_(is2p2))corresponding to the optical signals ω_(s1), ω_(s2).

But such supplementary couple of idlers are not so dramatic as theinitial FWM radiation since those idlers are generated outside theprincipal bands where the WDM optical signals are located. Therefore, nocross-talk can take place between the optical signals and those idlers.

In this case, the cut-off wavelengths of the couplers 5 to 8 must bechosen to correspond to the inverse of (•_(p1)+ω_(p2))/2. This time, itmay be judicious to chose it exactly to that value. The same alternativeimplementation as proposed in the case of a single pump radiation atFIGS. 2 and 3 can be also applied for the case of the use of two pumpradiations. On FIG. 5 is shown the implementation with symmetricinjection of the two pump radiations i.e. from the input 2 as well asthe output 3 of the optical device. On FIG. 6 is shown an alternativeimplementation where pump radiations are injected only by one end, herethe output 3. In that case, two mirrors 25, 26 must be inserted on thepath 9 flanking the non-linear optical medium 4. Such two mirrors 25, 26preferably but not exclusively chosen as two Fabry-Perot gratings mustbe centered at the wavelengths corresponding respectively to each pumpradiations to ensure co and contra-propagation of the pump radiations.Due to the different situation when using two pump radiations of carrierangular frequencies ω_(p1) and ω_(p2) with for each optical signal atcarrier angular frequency ω_(sj) the generation of two idlers one lessthan ω_(p1) and the other bigger than ω_(p2), supplementary couplers canbe used at the input 2 and output 3 of the optical device. On FIG. 7 isshown an optical device according to the invention with the opticalcircuit 1 as on FIG. 1 and two supplementary couplers 16, 18 and 20, 22respectively at its input 2 and output 3. The couplers 16, 20 and 18, 22can be chosen as multiplexers with a cut-off wavelength correspondingrespectively to the carrier angular frequency ω_(c1) and ω_(c2) asdefined on FIG. 8 a. In such a way, it is possible to couple first pumpradiation of carrier angular frequency ω_(p1), 17, 21, at coupler 16, 20and second optical radiation of carrier angular frequency ω_(p2), 19,23, at couplers 18, 22 with the WDM channels of the optical signals tobe amplified. The judicious choice of the cut-off wavelengths of the twocouplers 20, 22 at the output of the optical device permits to collectthe amplified optical signals solely without any idlers (ω_(is1p1),ω_(is1p2)), (ω_(is2p1), ω_(is2p2)).

All these solutions according to the invention enable to double theuseable amplification bandwidth. Furthermore, they do not depend on thechannel spacing and the number of used channels. Therefore, it has thebig advantage not to be limited when an optical device according to theinvention is already implemented onto an optical system while the numberof used channels is increased as long as the channels are within the twoprinciple amplification bands defined by the used optical parametricamplification and the cut-off of the couplers.

1. A method for amplifying wavelength division multiplexed opticalsignals by using an optical device comprising a non-linear opticalmedium for optical parametric amplification of the optical signal whentransmitted co-linearly with some pump radiation, the optical signalshaving carrier angular frequencies within the two principalamplification bands defined by the used optical parametricamplification, the method being whereby launching into the non-linearoptical medium in one direction the optical signals of carrier angularfrequencies within the one amplification band and in the oppositecounter-propagating direction the optical signals of carrier angularfrequencies within the other amplification band, the required pumpradiation being launched co-linearly with the respective optical signalsto be amplified.
 2. The method according to claim 1 whereby usingwavelength specific couplers with three ports for the optical device toseparate optical signals into two different paths of an optical circuitcomprising the non-linear optical medium while the separation beingperformed according to the carrier angular frequency property of theoptical signals if belonging within the first or the second principalamplification bands.
 3. The method according to claim 2 wherebycollecting at the output of the optical device the amplified opticalsignals while the unwanted produced four wave mixing radiation withinthe two principal amplification bands leaving the optical device by itsinput side.
 4. The method according to claim 3 whereby usingsupplementary wavelength specific couplers at the output of the opticalcircuit with a wavelength cut-off defined according to the respectivelyused pump radiations to be filtered out allowing to collect at theoutput of the optical device the amplified optical signal withoutunwanted produced idlers.
 5. The method according to claim 2 wherebycouplers with a wavelength cut-off for the separation limit beingdefined closed but not equal to the required pump radiation in the caseof degenerated optical parametric amplification.
 6. The method accordingto claim 1 whereby launching into the optical device the pump radiationin a symmetric way from its input as well as output side.
 7. An opticaldevice comprising: an optical circuit with at one arm a non-linearoptical medium for generating amplified optical signals having carrierangular frequencies within the two principal amplification bands definedby the used optical parametric amplification; wavelength specificcouplers with three ports to distribute optical signals according totheir carrier angular frequency property to different arms of theoptical circuit; wherein the wavelength specific couplers aredistributed over the optical circuit to launch into the non-linearoptical medium in one direction the optical signals having carrierangular frequencies within one of the two principal amplification bandand in the opposite counter-propagating direction the optical signals ofcarrier angular frequencies within the other amplification band, therequired pump radiation being launched co-linearly with the respectiveoptical signals to be amplified.
 8. The optical device according toclaim 7 wherein it comprises four such couplers defining an eighth-likeoptical circuit made out of two closed paths, one connected to the inputof the optical device by one of such couplers, the other being connectedto the output of the optical device by another coupler while both closedpaths having a common arm comprising the non-linear optical medium andbeing interconnected with the two closed paths at both ends by arespective coupler to collect at the output of the optical device theamplified optical signals while the unwanted produced four wave mixingradiations within the two amplification bands led to its input.
 9. Theoptical device according to claim 8 wherein it comprises mirror on thearm with the non-linear optical medium, the mirror being wavelengthspecific reflecting the required pump radiation to propagate in both waythrough the non-linear optical medium while being launched only at oneof the two ports from the optical device.
 10. The optical deviceaccording to claim 8 wherein it comprises supplementary couplers at theoutput of the optical circuit, the couplers having a wavelength cut-offdefined according to the respectively required pump radiation to befiltered out to collect at the output of the optical device theamplified optical signals without unwanted produced idlers.
 11. Theoptical device according to claim 7 wherein the couplers are defined bya wavelength cut-off for the separation limit being closed but not equalto the required pump radiation in the case of degenerated opticalparametric amplification.